The present invention is directed generally to a fluidized-bed combustion apparatus, and more particularly, to such an apparatus wherein a flow-reversing arrangement considerably improves combustion efficiency.
Of the various types of furnaces or combustors available, those utilizing fluidized-bed combustion techniques are of higher efficiency. In a typical fluidized-bed combustion apparatus particulate fuel such as coal or another combustible material is introduced into the combustion chamber and fluidized by a combustion-supporting medium passed through an underlying distributor plate or grid. Volatile gases and combustible particulate material evolved from the fluidized bed flow upwardly into a freeboard area overlying the fluidized bed where the combustion of the gaseous volatiles and the combustible particulate material continues until substantially all combustible material is consumed. Another advantage particular to fluidized-bed combustion is that the production of environmental pollutants such as sulfur oxides and various forms of nitrogen oxides is considerably reduced over that generated in other types of combustion apparatus.
While fluidized-bed combustors provide an advancement over the prior art combustor constructions, there are some shortcomings which detract from the overall efficiency of the fluidized-bed combustion techniques. For example, combustion efficiency is reduced due to the loss of combustibles in the spent bed material and the flue gas in the forms of unburned coal chars, carbon monoxide, and the like. Also, in conventional fluidized-bed combustors relatively high freeboard construction is required for attaining the desired combustion efficiency since both an adequate residence time and high temperatures are required in the freeboard volume to effect the combustion of the particulate material driven into the freeboard from the fluidized bed. The lack of a sufficient residence time and a sufficiently high freeboard temperature have been found to be major causes for losses in combustion efficiency which are primarily due to the non-combustion of the carbon-bearing particulate material driven into the freeboard. To provide the desired residence time the freeboard height was increased but even with good insulation techniques the temperature range with higher freeboards was hard to maintain at a sufficiently high level for the combustion of carbon-bearing fine particulate material in the freeboard. Further, the temperatures required in the freeboard area to effect the reduction of the various nitrogen oxides were difficult to maintain. In fact, in a fluidized-bed combustion operation the major heat source is from the burning of the fluidized-bed material which is located a considerable distance from the uppermost portion of the freeboard. Also, the shielding action by the solid particles and the gaseous products of combustion in the freeboard contribute significantly to the lack of control of the freeboard temperature so as to decrease the efficiency of the fluidized-bed combustion apparatus.